Q: Q2. Write the names of isotopes of hydrogen. What is the mass ratio of these isotopes?

Hydrogen has three isotopes:\n\n1. **Protium (¹H or H):** It has one proton and no neutrons. Its atomic mass is approximately 1 amu.\n2. **Deuterium (²H or D):** It has one proton and one neutron. Its atomic mass is approximately 2 amu. It is also known as heavy hydrogen.\n3. **Tritium (³H or T):** It has one proton and two neutrons. Its atomic mass is approximately 3 amu. It is a radioactive isotope with a half-life of 12.33 years.\n\n**Mass Ratio of these isotopes:**\nThe approximate mass ratio of Protium : Deuterium : Tritium is 1 : 2 : 3.

Q: Q6. What do you understand by the term 'non-stoichiometric hydrides'? Do you expect this type of hydride to be formed by alkali metals? Justify your answer.

### Non-stoichiometric Hydrides\nNon-stoichiometric hydrides are metallic hydrides that do not obey the law of constant proportions. This means the ratio of hydrogen to the metal atom is not a simple whole number, and their composition varies within a certain range. They are typically formed by d-block and f-block elements (Groups 7, 8, 9 do not form hydrides, and Group 6 forms only CrH). These hydrides are often called interstitial hydrides because hydrogen atoms occupy the interstitial sites (voids) in the metal lattice without significantly altering the metal's crystal structure. The exact composition depends on the temperature and pressure of hydrogen during their formation. Examples include TiH1.5-1.8, ZrH1.3-1.75, VH0.56, LaH2.76, etc.\n\n### Formation by Alkali Metals\nNo, we do not expect non-stoichiometric hydrides to be formed by alkali metals.\n\n**Justification:**\nAlkali metals (Group 1 elements) are highly electropositive and react with hydrogen to form ionic (saline) hydrides, such as LiH, NaH, KH, RbH, and CsH. In these hydrides, hydrogen exists as a hydride ion (H⁻), and the metal exists as a cation (M⁺). These hydrides are stoichiometric compounds, meaning they have a fixed and definite composition (M:H ratio is always 1:1). They are salt-like, crystalline solids with high melting points and conduct electricity in the molten state. The formation of ionic bonds leads to a fixed stoichiometry, unlike the interstitial nature of non-stoichiometric hydrides where hydrogen atoms are accommodated in the voids of the metal lattice.

Question 1

Q1. Discuss the position of hydrogen in the periodic table.

Accepted Answer

Hydrogen occupies a unique position in the periodic table due to its distinctive properties. Its electronic configuration is 1s¹, which means it has only one electron in its outermost shell. This configuration leads to properties similar to both alkali metals (Group 1) and halogens (Group 17), as well as some unique characteristics.

### Similarities with Alkali Metals (Group 1)
1.  **Electronic Configuration:** Like alkali metals (e.g., Li: [He]2s¹, Na: [Ne]3s¹), hydrogen has one electron in its outermost shell. 2.  **Electropositive Character:** Hydrogen can lose its single electron to form a unipositive ion (H⁺), similar to alkali metals which form M⁺ ions (e.g., Na⁺, K⁺). 3.  **Oxidation State:** It exhibits a +1 oxidation state in compounds with more electronegative elements (e.g., HCl, H₂O), just like alkali metals. 4.  **Combination with Electronegative Elements:** Hydrogen combines with electronegative elements like halogens, oxygen, and sulfur to form compounds (e.g., HCl, H₂O, H₂S), similar to alkali metals (e.g., NaCl, Na₂O, Na₂S).

### Similarities with Halogens (Group 17)
1.  **Electronic Configuration:** Hydrogen requires only one electron to complete its K-shell (to achieve a stable duplet configuration like helium, 1s²), similar to halogens which require one electron to complete their outermost octet (e.g., F: [He]2s²2p⁵ needs one electron to become [He]2s²2p⁶). 2.  **Electronegative Character:** Hydrogen can gain an electron to form a uninegative ion (H⁻), called the hydride ion, similar to halogens which form halide ions (X⁻). This occurs when hydrogen reacts with highly electropositive metals (e.g., NaH, CaH₂). 3.  **Diatomic Nature:** Hydrogen exists as a diatomic molecule (H₂) in its elemental state, similar to halogens (F₂, Cl₂, Br₂, I₂). 4.  **Ionization Enthalpy:** The ionization enthalpy of hydrogen (1312 kJ/mol) is quite high, comparable to that of halogens (e.g., F: 1680 kJ/mol, Cl: 1251 kJ/mol) and much higher than alkali metals (e.g., Li: 520 kJ/mol, Na: 496 kJ/mol).

### Dissimilarities and Unique Position
1.  **Absence of Neutrons:** Protium (¹H), the most common isotope of hydrogen, does not contain any neutrons in its nucleus, unlike any other element. 2.  **Small Size:** The H⁺ ion is extremely small (a bare proton) and does not exist freely in aqueous solutions but is always associated with other atoms or molecules (e.g., H₃O⁺). Alkali metal ions (M⁺) are much larger. 3.  **Metallic Character:** Hydrogen is a non-metal, while alkali metals are highly metallic. 4.  **Electronegativity:** Hydrogen's electronegativity (2.20 on Pauling scale) is intermediate, allowing it to form both ionic and covalent bonds. Alkali metals are highly electropositive, and halogens are highly electronegative.

### Conclusion
Due to these dual similarities and unique properties, hydrogen cannot be placed definitively with either alkali metals or halogens. It is typically placed separately at the top of the periodic table, often above Group 1, acknowledging its unique position as the first element.

Question 2

Q2. Write the names of isotopes of hydrogen. What is the mass ratio of these isotopes?

Accepted Answer

Hydrogen has three isotopes:

1.  **Protium (¹H or H):** It has one proton and no neutrons. Its atomic mass is approximately 1 amu.
2.  **Deuterium (²H or D):** It has one proton and one neutron. Its atomic mass is approximately 2 amu. It is also known as heavy hydrogen.
3.  **Tritium (³H or T):** It has one proton and two neutrons. Its atomic mass is approximately 3 amu. It is a radioactive isotope with a half-life of 12.33 years.

**Mass Ratio of these isotopes:**
The approximate mass ratio of Protium : Deuterium : Tritium is 1 : 2 : 3.

Question 3

Q3. Why does hydrogen exhibit dual behaviour (i.e. as a reductant and as an oxidant)?

Accepted Answer

Hydrogen exhibits dual behaviour, acting both as a reductant (reducing agent) and an oxidant (oxidizing agent), primarily due to its electronic configuration (1s¹) and its intermediate electronegativity.

1.  **As a Reductant (Reducing Agent):**
    *   Hydrogen has only one electron in its outermost shell. It can readily lose this electron to form a stable unipositive ion (H⁺), achieving the stable electronic configuration of the noble gas helium (1s⁰, effectively).     *   When hydrogen loses an electron, its oxidation state changes from 0 to +1. This process is oxidation for hydrogen, and it causes the reduction of another species.     *   Therefore, hydrogen acts as a reductant when it reacts with highly electronegative elements like halogens (e.g., F₂, Cl₂) or oxygen.
    *   **Example:** H₂ + Cl₂ → 2HCl (Hydrogen reduces Cl₂ to Cl⁻)

2.  **As an Oxidant (Oxidizing Agent):**
    *   Hydrogen can also gain one electron to complete its K-shell and achieve a stable duplet configuration (1s²), similar to helium.     *   When hydrogen gains an electron, its oxidation state changes from 0 to -1, forming the hydride ion (H⁻). This process is reduction for hydrogen, and it causes the oxidation of another species.     *   Therefore, hydrogen acts as an oxidant when it reacts with highly electropositive metals (alkali metals and alkaline earth metals) to form ionic hydrides.
    *   **Example:** 2Na + H₂ → 2NaH (Hydrogen oxidizes Na to Na⁺)

Question 4

Q4. What is the difference between ortho and para hydrogen?

Accepted Answer

The difference between ortho and para hydrogen lies in the relative orientation of the nuclear spins of the two hydrogen atoms in a hydrogen molecule (H₂).

1.  **Ortho Hydrogen:**
    *   In ortho hydrogen, the nuclear spins of the two hydrogen atoms are in the **same direction** (parallel).     *   It is the higher energy and more stable form at higher temperatures.     *   At room temperature (around 25°C), ordinary hydrogen is approximately 75% ortho hydrogen and 25% para hydrogen.

2.  **Para Hydrogen:**
    *   In para hydrogen, the nuclear spins of the two hydrogen atoms are in the **opposite direction** (anti-parallel).     *   It is the lower energy and more stable form at very low temperatures.     *   As the temperature decreases, the equilibrium shifts towards para hydrogen. At absolute zero (0 K), hydrogen consists almost entirely of para hydrogen.

**Key Differences Summarized:**

Question 5

Q5. Why is hydrogen not found in nature as a free element?

Accepted Answer

Hydrogen is not found in nature as a free element (H₂) in significant quantities, especially in the Earth's atmosphere, primarily due to its high reactivity and low atomic mass.

1.  **High Reactivity:** Hydrogen has a strong tendency to react with other elements. Its electronic configuration (1s¹) means it needs only one electron to achieve a stable duplet (like helium) or can lose one electron to form a stable H⁺ ion. This makes it highly reactive and readily form chemical bonds with other elements. It combines with oxygen to form water, with carbon to form hydrocarbons, and with other elements to form various compounds.

2.  **Low Ionization Enthalpy:** Although its ionization enthalpy is high compared to alkali metals, it is still low enough for it to readily lose its electron and participate in chemical reactions, especially with highly electronegative elements.

3.  **Lightness and Escape Velocity:** Being the lightest element, if any free hydrogen were present in the Earth's atmosphere, its molecules would have very high kinetic energy at ambient temperatures. This would allow them to easily attain escape velocity and escape into outer space, preventing its accumulation in the atmosphere.

Therefore, hydrogen is predominantly found in combined forms, such as water (H₂O), acids, bases, hydrocarbons, and other organic and inorganic compounds, rather than as a free element.

Question 6

Q6. What do you understand by the term 'non-stoichiometric hydrides'? Do you expect this type of hydride to be formed by alkali metals? Justify your answer.

Accepted Answer

### Non-stoichiometric Hydrides
Non-stoichiometric hydrides are metallic hydrides that do not obey the law of constant proportions. This means the ratio of hydrogen to the metal atom is not a simple whole number, and their composition varies within a certain range. They are typically formed by d-block and f-block elements (Groups 7, 8, 9 do not form hydrides, and Group 6 forms only CrH). These hydrides are often called interstitial hydrides because hydrogen atoms occupy the interstitial sites (voids) in the metal lattice without significantly altering the metal's crystal structure. The exact composition depends on the temperature and pressure of hydrogen during their formation. Examples include TiH1.5-1.8, ZrH1.3-1.75, VH0.56, LaH2.76, etc.

### Formation by Alkali Metals
No, we do not expect non-stoichiometric hydrides to be formed by alkali metals.

**Justification:**
Alkali metals (Group 1 elements) are highly electropositive and react with hydrogen to form ionic (saline) hydrides, such as LiH, NaH, KH, RbH, and CsH. In these hydrides, hydrogen exists as a hydride ion (H⁻), and the metal exists as a cation (M⁺). These hydrides are stoichiometric compounds, meaning they have a fixed and definite composition (M:H ratio is always 1:1). They are salt-like, crystalline solids with high melting points and conduct electricity in the molten state. The formation of ionic bonds leads to a fixed stoichiometry, unlike the interstitial nature of non-stoichiometric hydrides where hydrogen atoms are accommodated in the voids of the metal lattice.

Question 7

Q7. How do you expect the metallic hydrides to be useful for hydrogen storage? Explain.

Accepted Answer

Metallic hydrides, particularly those formed by certain d-block and f-block metals (e.g., Pd, Ti, Zr, La, Mg, Ni), show great promise for hydrogen storage due to their unique properties. Here's how they are useful:

1.  **High Storage Capacity:** Many metals can absorb a large volume of hydrogen, often exceeding the volume of the metal itself. For example, palladium can absorb up to 900 times its own volume of hydrogen. This allows for compact storage of hydrogen.

2.  **Safety:** Hydrogen gas is highly flammable and explosive when mixed with air. Storing it as a solid metallic hydride is much safer than storing it as a compressed gas or a cryogenic liquid. The hydrogen is chemically bound within the metal lattice, reducing the risk of leaks and explosions.

3.  **Reversible Absorption/Desorption:** The process of hydrogen absorption by metals to form hydrides is often reversible. By varying temperature and pressure, hydrogen can be absorbed into the metal lattice (forming the hydride) and subsequently released from it. For example, heating the metallic hydride causes it to decompose and release hydrogen gas, while cooling and increasing hydrogen pressure facilitates absorption.

*   **Absorption:** M(s) + x/2 H₂(g) ⇌ MHₓ(s) + Heat
    *   **Desorption:** MHₓ(s) + Heat ⇌ M(s) + x/2 H₂(g)

4.  **Controlled Release:** The rate of hydrogen release can be controlled by adjusting the temperature and pressure, making it suitable for applications where a steady supply of hydrogen is required, such as in fuel cells.

5.  **Lower Pressure Storage:** Unlike compressed hydrogen gas cylinders which require very high pressures (hundreds of bars), hydrogen can be stored in metallic hydrides at relatively lower pressures, reducing the engineering challenges and costs associated with high-pressure containment.

**Mechanism:** Hydrogen molecules (H₂) first dissociate into atomic hydrogen on the metal surface. These hydrogen atoms then diffuse into the interstitial sites (voids) within the metal crystal lattice, forming the metallic hydride. This process is often exothermic. The ability of these metals to reversibly absorb and desorb hydrogen makes them excellent candidates for solid-state hydrogen storage, which is crucial for the development of hydrogen-powered vehicles and other clean energy technologies.

Question 8

Q8. What are the consequences of temporary and permanent hardness of water?

Accepted Answer

Hardness of water is caused by the presence of dissolved mineral salts, primarily bicarbonates, chlorides, and sulfates of calcium and magnesium. It is broadly classified into temporary and permanent hardness, each with distinct consequences.

### Consequences of Temporary Hardness
Temporary hardness is caused by the presence of dissolved bicarbonates of calcium (Ca(HCO₃)₂) and magnesium (Mg(HCO₃)₂). It can be removed by boiling.

1.  **Scaling in Boilers and Hot Water Systems:** When hard water is heated, the soluble bicarbonates decompose to form insoluble carbonates (CaCO₃ and MgCO₃), which precipitate out as a hard scale (limescale) on the inner surfaces of boilers, kettles, and hot water pipes.
    *   Ca(HCO₃)₂(aq) → CaCO₃(s) + H₂O(l) + CO₂(g)
    *   Mg(HCO₃)₂(aq) → MgCO₃(s) + H₂O(l) + CO₂(g)
    This scale reduces heat transfer efficiency, leading to increased fuel consumption and potential overheating of the boiler tubes, which can cause them to burst. It also narrows pipes, reducing water flow.

2.  **Wastage of Soap:** Hard water reacts with soap (sodium stearate, C₁₇H₃₅COONa) to form an insoluble precipitate called 'scum' (calcium or magnesium stearate). This scum does not lather and adheres to clothes, skin, and plumbing fixtures. A significant amount of soap is wasted in precipitating the hardness-causing ions before lather can be formed.
    *   2C₁₇H₃₅COONa(aq) + Ca²⁺(aq) → (C₁₇H₃₅COO)₂Ca(s) + 2Na⁺(aq)

3.  **Poor Washing Quality:** The scum formed with soap leaves stains and dulls clothes. It can also clog pores on the skin and make hair feel sticky.

4.  **Taste:** While not always negative, some people find the taste of temporarily hard water less palatable due to the dissolved minerals.

### Consequences of Permanent Hardness
Permanent hardness is caused by the presence of dissolved chlorides (CaCl₂, MgCl₂) and sulfates (CaSO₄, MgSO₄) of calcium and magnesium. It cannot be removed by boiling.

1.  **Scaling in Boilers and Industrial Equipment:** Similar to temporary hardness, permanent hardness also leads to the formation of hard scales in boilers, heat exchangers, and industrial equipment. Calcium sulfate (CaSO₄) is particularly problematic as it forms a very hard, adherent scale that is difficult to remove. This leads to reduced efficiency, increased maintenance costs, and potential equipment damage.

2.  **Wastage of Soap:** Permanent hardness also causes significant wastage of soap, as the Ca²⁺ and Mg²⁺ ions react with soap to form insoluble scum, preventing lather formation until all hardness ions are precipitated.

3.  **Corrosion:** Chlorides and sulfates can increase the corrosivity of water, leading to accelerated corrosion of pipes and metallic equipment, especially at higher temperatures.

4.  **Health Issues (Minor):** While generally safe for drinking, extremely hard water can sometimes contribute to digestive problems in sensitive individuals. However, moderate hardness is often considered beneficial due to the presence of essential minerals like calcium and magnesium.

In summary, both types of hardness lead to economic losses due to increased fuel consumption, soap wastage, equipment damage, and maintenance, making water softening an essential process for many industrial and domestic applications.

Question 9

Q9. Can we use ozonised oxygen in the titration of KMnO4 solution? If not, why?

Accepted Answer

No, we cannot use ozonised oxygen (a mixture containing ozone, O₃) in the titration of KMnO₄ solution.

**Justification:**
Both ozone (O₃) and potassium permanganate (KMnO₄) are strong oxidizing agents. In a titration, one substance acts as an oxidizing agent and the other as a reducing agent. If both are strong oxidizing agents, they will not react in a controlled and stoichiometric manner that is suitable for titration. Instead, they might react with each other or with other components in the solution in complex ways, leading to inaccurate results. For a successful titration, a clear redox reaction between a strong oxidizing agent and a strong reducing agent (or vice versa) is required, where one is titrated against the other to determine its concentration.

Question 10

Q10. Write chemical reactions to show the amphoteric nature of water.

Accepted Answer

Water is an amphoteric substance, meaning it can act as both an acid and a base. This behavior can be explained using the Brønsted-Lowry theory of acids and bases.

1.  **Water acting as an acid (proton donor):**
    When water reacts with a stronger base (like ammonia, NH₃), it donates a proton (H⁺) and acts as an acid, forming its conjugate base (hydroxide ion, OH⁻).
    
    H₂O(l) + NH₃(aq) ⇌ OH⁻(aq) + NH₄⁺(aq)
    (Acid)    (Base)      (Conjugate Base) (Conjugate Acid)

2.  **Water acting as a base (proton acceptor):**
    When water reacts with a stronger acid (like hydrogen sulfide, H₂S), it accepts a proton (H⁺) and acts as a base, forming its conjugate acid (hydronium ion, H₃O⁺).
    
    H₂O(l) + H₂S(aq) ⇌ H₃O⁺(aq) + HS⁻(aq)
    (Base)    (Acid)      (Conjugate Acid) (Conjugate Base)

3.  **Auto-protolysis (Self-ionization) of water:**
    Water can also react with itself, where one water molecule acts as an acid and another acts as a base. This is the basis for the pH scale.
    
    H₂O(l) + H₂O(l) ⇌ H₃O⁺(aq) + OH⁻(aq)
    (Acid)    (Base)      (Conjugate Acid) (Conjugate Base)

Question 11

Q11. Describe the industrial production of dihydrogen.

Accepted Answer

Dihydrogen (H₂) is produced industrially by several methods, primarily to meet the large demand for it in various chemical processes. The main industrial methods include:

### 1. Electrolysis of Acidified Water
This method is used for the production of high purity dihydrogen, especially when oxygen is also required. It is an energy-intensive process.

**Process:**
*   Water is a poor conductor of electricity, so a small amount of acid (like H₂SO₄) or base (like NaOH) is added to make it conductive.
*   Direct current is passed through the acidified water.
*   Hydrogen gas is liberated at the cathode, and oxygen gas at the anode.

**Reactions:**
*   **At cathode:** 2H₂O(l) + 2e⁻ → H₂(g) + 2OH⁻(aq)
*   **At anode:** 2H₂O(l) → O₂(g) + 4H⁺(aq) + 4e⁻
*   **Overall reaction:** 2H₂O(l) → 2H₂(g) + O₂(g)

### 2. Steam Reforming of Hydrocarbons (Bosch Process)
This is the most common and economical method for large-scale production of dihydrogen, especially from natural gas (methane) or naphtha.

**Process:**
*   **Step 1: Steam Reforming:** Hydrocarbons (e.g., methane) are reacted with steam at high temperatures (773-1273 K) in the presence of a nickel catalyst.
    CH₄(g) + H₂O(g) \xrightarrow{1270 K, Ni} CO(g) + 3H₂(g)
    (This mixture of CO and H₂ is called 'synthesis gas' or 'syngas').
*   **Step 2: Water-Gas Shift Reaction:** The carbon monoxide produced in the first step is further reacted with steam to produce more dihydrogen. This reaction is called the water-gas shift reaction.
    CO(g) + H₂O(g) \xrightarrow{673 K, FeCrO₄ catalyst} CO₂(g) + H₂(g)
*   **Step 3: Removal of CO₂:** Carbon dioxide is removed by scrubbing with water under pressure (to form carbonic acid) or by absorption in a solution of potassium carbonate.

### 3. Coal Gasification
This method involves reacting steam with hot coke (from coal) to produce synthesis gas, which is then subjected to the water-gas shift reaction.

**Process:**
*   **Step 1: Coal Gasification:** Hot coke reacts with steam at high temperatures (1270 K).
    C(s) + H₂O(g) \xrightarrow{1270 K} CO(g) + H₂(g)
    (This mixture is also known as 'water gas').
*   **Step 2: Water-Gas Shift Reaction:** Similar to the hydrocarbon reforming, the CO is converted to CO₂ and more H₂.
    CO(g) + H₂O(g) \xrightarrow{673 K, FeCrO₄ catalyst} CO₂(g) + H₂(g)
*   **Step 3: Removal of CO₂:** CO₂ is removed as described above.

### 4. From Brine Electrolysis (Chlor-alkali Process)
Dihydrogen is obtained as a byproduct during the electrolysis of brine (aqueous NaCl solution) for the production of chlorine and sodium hydroxide.

**Reactions:**
*   **At cathode:** 2H₂O(l) + 2e⁻ → H₂(g) + 2OH⁻(aq)
*   **At anode:** 2Cl⁻(aq) → Cl₂(g) + 2e⁻
*   **Overall reaction:** 2NaCl(aq) + 2H₂O(l) → 2NaOH(aq) + Cl₂(g) + H₂(g)

Among these, steam reforming of hydrocarbons and coal gasification are the most common large-scale methods.

Question 12

Q12. Give the reactions that take place in the Bosch process for the production of dihydrogen from steam and coke.

Accepted Answer

The Bosch process for the production of dihydrogen from steam and coke involves two main steps:

### 1. Coal Gasification (Formation of Water Gas)
Hot coke (carbon) reacts with steam at very high temperatures (around 1270 K) to produce a mixture of carbon monoxide and dihydrogen. This mixture is known as 'water gas'.

**Reaction:**
C(s) + H₂O(g) \xrightarrow{1270 K} CO(g) + H₂(g)
(Coke) + (Steam) → (Carbon monoxide) + (Dihydrogen)

### 2. Water-Gas Shift Reaction
To increase the yield of dihydrogen, the carbon monoxide produced in the first step is further reacted with more steam. This reaction is carried out at a lower temperature (around 673 K) in the presence of an iron chromate (FeCrO₄) catalyst. Carbon monoxide is oxidized to carbon dioxide, and more dihydrogen is produced.

**Reaction:**
CO(g) + H₂O(g) \xrightarrow{673 K, FeCrO₄ catalyst} CO₂(g) + H₂(g)
(Carbon monoxide) + (Steam) → (Carbon dioxide) + (Dihydrogen)

After these two steps, the carbon dioxide is typically removed by scrubbing with water under pressure or by absorption in a solution of potassium carbonate, leaving behind pure dihydrogen.

Question 13

Q13. With the help of suitable examples, explain the difference between electron-deficient, electron-precise and electron-rich hydrides.

Accepted Answer

Covalent hydrides are formed by elements of p-block (and some s-block elements like Be and Mg). Based on the number of electrons available for bonding and the presence of lone pairs, covalent hydrides can be classified into three types:

### 1. Electron-Deficient Hydrides
*   **Definition:** These hydrides have too few electrons to form normal covalent bonds (i.e., to complete their octet) according to the classical Lewis structure representation. They have fewer electrons than required to form the expected number of covalent bonds.
*   **Characteristics:**
    *   They act as Lewis acids (electron acceptors) because they have vacant orbitals.
    *   They tend to accept electron pairs from other molecules to complete their octet.
    *   They often exist as polymeric structures or form bridge bonds (e.g., three-centre two-electron bonds) to achieve stability.
*   **Formation:** Formed by Group 13 elements.
*   **Examples:** Boranes like diborane (B₂H₆) is the simplest example. BH₃ is electron-deficient and dimerizes to B₂H₆. Other examples include AlH₃.

### 2. Electron-Precise Hydrides
*   **Definition:** These hydrides have the exact number of electrons required to form conventional covalent bonds (i.e., to complete their octet) without any lone pairs on the central atom.
*   **Characteristics:**
    *   They have a tetrahedral geometry (sp³ hybridization) around the central atom.
    *   They are generally stable and do not act as Lewis acids or bases.
*   **Formation:** Formed by Group 14 elements.
*   **Examples:** Methane (CH₄), silane (SiH₄), germane (GeH₄).

### 3. Electron-Rich Hydrides
*   **Definition:** These hydrides have more electrons than required to form conventional covalent bonds, meaning they have one or more lone pairs of electrons on the central atom.
*   **Characteristics:**
    *   Due to the presence of lone pairs, they act as Lewis bases (electron donors).
    *   The lone pairs can participate in hydrogen bonding, which significantly affects their physical properties (e.g., higher boiling points).
*   **Formation:** Formed by Group 15, 16, and 17 elements.
*   **Examples:**
    *   **Group 15:** Ammonia (NH₃) - one lone pair on nitrogen.
    *   **Group 16:** Water (H₂O) - two lone pairs on oxygen.
    *   **Group 17:** Hydrogen fluoride (HF) - three lone pairs on fluorine.

In summary, the classification depends on whether the central atom in the hydride has fewer, exactly enough, or more valence electrons than needed to achieve a stable octet through covalent bonding.

Question 14

Q14. How do you expect the metallic hydrides to be useful for hydrogen storage? Explain.

Accepted Answer

Metallic hydrides, also known as interstitial hydrides, are formed by many d-block and f-block elements. They are expected to be useful for hydrogen storage due to their unique ability to reversibly absorb and release large volumes of hydrogen.

**Explanation:**
1.  **Absorption of Hydrogen:** Many transition metals and rare earth metals have the ability to absorb hydrogen atoms into the interstitial sites (voids) within their crystal lattice. This process is often exothermic and occurs under specific temperature and pressure conditions. The hydrogen atoms dissociate into protons and electrons, which then occupy the interstitial spaces, forming a metallic hydride (e.g., LaNi₅H₆, MgH₂, TiH₂).
2.  **High Storage Capacity:** These metallic hydrides can store a much larger volume of hydrogen per unit volume compared to compressed hydrogen gas or even liquid hydrogen. For example, some hydrides can store hydrogen at densities comparable to or even higher than liquid hydrogen.
3.  **Reversible Release of Hydrogen:** The absorbed hydrogen can be released by heating the metallic hydride or by reducing the pressure. This reversibility is crucial for practical storage applications. The hydrogen is released as H₂ gas, making it available for use.
4.  **Safety:** Storing hydrogen as a solid hydride is inherently safer than storing it as a highly flammable compressed gas or cryogenic liquid. The risk of explosion or leakage is significantly reduced.
5.  **Compactness:** The ability to store hydrogen in a solid state allows for more compact storage systems compared to bulky high-pressure gas cylinders.

**Challenges and Future Potential:**
Despite these advantages, there are challenges:
*   **Weight:** Many metallic hydrides are heavy, which is a disadvantage for mobile applications (e.g., fuel cell vehicles).
*   **Cost:** The cost of some suitable metals or alloys can be high.
*   **Kinetics:** The rates of hydrogen absorption and desorption can sometimes be slow.
*   **Cycling Stability:** Repeated absorption-desorption cycles can sometimes degrade the material.

However, ongoing research is focused on developing lighter, cheaper, and more efficient metallic hydride materials with improved kinetics and cycling stability. Their potential for safe, compact, and efficient hydrogen storage makes them a promising solution for the 'hydrogen economy', particularly for fuel cell vehicles and stationary power applications.

Question 15

Q15. What do you understand by the term 'non-stoichiometric hydrides'? Do you expect this type of hydrides to be formed by alkali metals? Justify your answer.

Accepted Answer

### What are Non-Stoichiometric Hydrides?
Non-stoichiometric hydrides are a type of metallic (or interstitial) hydrides that do not obey the law of constant proportions. This means the ratio of the metal to hydrogen atoms in these compounds is not a simple whole number and can vary within a certain range, depending on the conditions of preparation. Their composition is typically non-integer, for example, TiH₁.₅₋₁.₈ or LaH₂.₈₇. This non-stoichiometry arises because hydrogen atoms occupy interstitial sites in the metal lattice, and not all available sites may be filled, or the filling can vary.

These hydrides are primarily formed by d-block (transition metals) and f-block (lanthanides and actinides) elements. They are often referred to as interstitial hydrides because hydrogen atoms occupy the interstitial voids in the metal lattice, leading to a distortion of the lattice without forming distinct chemical bonds in the traditional sense.

### Do alkali metals form non-stoichiometric hydrides? Justify your answer.
No, alkali metals (Group 1 elements like Li, Na, K, Rb, Cs) do not form non-stoichiometric hydrides.

**Justification:**
Alkali metals form **ionic hydrides** (also known as saline hydrides). In these hydrides, the alkali metal atom donates its single valence electron to the hydrogen atom, forming a metal cation (M⁺) and a hydride anion (H⁻). These compounds are typically crystalline, non-volatile solids with a definite stoichiometry (e.g., LiH, NaH, KH).

*   **Ionic Nature:** The bonding in alkali metal hydrides is predominantly ionic, involving a complete transfer of electrons. This leads to fixed valencies and definite stoichiometric ratios.
*   **Fixed Stoichiometry:** Due to the strong ionic bonding and the fixed valency of alkali metals (+1) and hydrogen (-1), these hydrides always have a precise 1:1 metal-to-hydrogen ratio (MH). They do not exhibit the variable composition characteristic of non-stoichiometric hydrides.
*   **Absence of Interstitial Sites:** Alkali metals do not have the suitable crystal structures or vacant d-orbitals to accommodate hydrogen atoms in interstitial positions in a variable manner, which is the basis for non-stoichiometry in d- and f-block metals.

Therefore, alkali metals form stoichiometric ionic hydrides, not non-stoichiometric hydrides.

Question 16

Q16. What is the difference between the terms 'hydrolysis' and 'hydration'?

Accepted Answer

The terms 'hydrolysis' and 'hydration' both involve water, but they describe distinct chemical processes:

**Hydrolysis:**
Hydrolysis is a chemical reaction in which a molecule is broken down by reaction with water. The word 'hydro' means water and 'lysis' means to break. In hydrolysis, water itself acts as a reactant, often splitting into H⁺ and OH⁻ ions, which then react with the substrate molecule, leading to its cleavage or decomposition. This process typically involves the breaking of a bond in the substrate molecule.

*   **Key characteristics:**
    *   Water acts as a reactant.
    *   It involves the breaking of chemical bonds in the substrate.
    *   It often results in the formation of two or more new compounds.
    *   Examples: Digestion of carbohydrates (e.g., sucrose hydrolysis into glucose and fructose), hydrolysis of esters, amides, salts of weak acids/bases (e.g., sodium acetate hydrolysis in water to produce acetic acid and NaOH).
    *   Example reaction: CH₃COOCH₃ (ester) + H₂O → CH₃COOH (acid) + CH₃OH (alcohol)

**Hydration:**
Hydration is a process where water molecules surround and interact with a solute or an ion, without necessarily breaking down the solute molecule. It is a type of solvation where water acts as the solvent. The water molecules form a hydration shell around the solute particles due to intermolecular forces (like ion-dipole interactions, hydrogen bonding, or dipole-dipole interactions). This process is primarily physical, though it can lead to the formation of hydrates (compounds containing water molecules).

*   **Key characteristics:**
    *   Water acts as a solvent.
    *   It involves the formation of intermolecular bonds (e.g., hydrogen bonds, ion-dipole interactions) between water and the solute.
    *   It typically does not involve the breaking of covalent bonds within the solute molecule.
    *   It is often an exothermic process, releasing energy (enthalpy of hydration).
    *   Examples: Dissolution of ionic compounds in water (e.g., NaCl dissolving in water, where Na⁺ and Cl⁻ ions get hydrated), hydration of cement, hydration of anhydrous salts to form hydrated salts (e.g., CuSO₄ + 5H₂O → CuSO₄·5H₂O).
    *   Example process: Na⁺(g) + nH₂O → Na⁺(aq)

**In summary:** Hydrolysis is a chemical reaction involving bond breaking by water, while hydration is a physical process of surrounding a solute with water molecules.

Question 17

Q17. How can saline hydrides react with water so as to produce hydrogen gas?

Accepted Answer

Saline hydrides (also known as ionic hydrides) are formed by the reaction of hydrogen with highly electropositive s-block elements (Group 1 and Group 2 metals, except beryllium). These hydrides are typically crystalline, non-volatile solids with high melting points. In these compounds, hydrogen exists as a hydride ion (H⁻), which is a very strong base and a powerful reducing agent.

When saline hydrides react with water, the highly reactive hydride ion (H⁻) readily accepts a proton (H⁺) from water, leading to the formation of hydrogen gas (H₂) and hydroxide ions (OH⁻). This reaction is highly exothermic and often vigorous, sometimes explosive, especially with alkali metal hydrides.

**Mechanism of Reaction:**
Water (H₂O) can be considered as H⁺—OH⁻. The hydride ion (H⁻) from the saline hydride acts as a strong base and abstracts a proton (H⁺) from water. This results in the formation of a hydrogen molecule (H₂) and leaves behind a hydroxide ion (OH⁻), which combines with the metal cation (M⁺) to form the metal hydroxide (MOH).

**General Reaction:**
MH(s) + H₂O(l) → MOH(aq) + H₂(g)

Where M represents an alkali metal (e.g., Li, Na, K) or an alkaline earth metal (e.g., Ca, Sr, Ba).

**Specific Examples:**
1.  **Lithium Hydride (LiH) with water:**
    LiH(s) + H₂O(l) → LiOH(aq) + H₂(g)

2.  **Sodium Hydride (NaH) with water:**
    NaH(s) + H₂O(l) → NaOH(aq) + H₂(g)

3.  **Calcium Hydride (CaH₂) with water:**
    CaH₂(s) + 2H₂O(l) → Ca(OH)₂(aq) + 2H₂(g)
    (Note: For alkaline earth metal hydrides, two H⁻ ions are present, requiring two molecules of water to produce two molecules of H₂ gas.)

This reaction is a convenient method for producing hydrogen gas in the laboratory, especially for small-scale generation, and is also utilized in portable hydrogen generators.

Question 18

Q18. Do you expect the carbon hydrides of type (CnH2n+2) to act as Lewis acid or base? Justify your answer.

Accepted Answer

Carbon hydrides of the type (CnH2n+2) are alkanes, which are saturated hydrocarbons. Let's analyze their structure and bonding to determine if they can act as Lewis acids or bases.

**Lewis Acid:** A Lewis acid is an electron-pair acceptor. It typically has an empty orbital to accept an electron pair or is electron-deficient.

**Lewis Base:** A Lewis base is an electron-pair donor. It typically has a lone pair of electrons or a region of high electron density that can be donated.

**Analysis of Alkanes (CnH2n+2):**
1.  **Bonding:** Alkanes consist only of carbon-carbon (C-C) and carbon-hydrogen (C-H) single covalent bonds. All carbon atoms are sp³ hybridized, forming four single bonds.
2.  **Electron Deficiency/Availability:**
    *   **No empty orbitals:** Carbon atoms in alkanes have completed their octet (four bonds, eight valence electrons). There are no empty valence orbitals available to accept electron pairs.
    *   **No lone pairs:** Neither carbon nor hydrogen atoms in alkanes possess any non-bonding (lone) pairs of electrons that can be donated.
    *   **Non-polar C-H bonds:** Although there is a slight electronegativity difference between carbon (2.55) and hydrogen (2.20), the C-H bonds are considered essentially non-polar. This means there are no significant partial positive charges on carbon or hydrogen that would make them attractive to electron pairs (as Lewis acids), nor are there highly localized electron densities that could be easily donated (as Lewis bases).

**Conclusion:**
Based on the above analysis, carbon hydrides of the type (CnH2n+2), i.e., alkanes, are neither Lewis acids nor Lewis bases under normal conditions.

*   They cannot act as Lewis acids because they do not have empty orbitals to accept electron pairs.
*   They cannot act as Lewis bases because they do not possess any lone pairs of electrons to donate.

Alkanes are generally unreactive and are often referred to as paraffins (from Latin 'parum affinis', meaning 'little affinity'). Their primary reactions involve combustion, free radical substitution, or cracking, which typically occur under harsh conditions and do not involve Lewis acid-base interactions in the conventional sense.

Question 19

Q19. What do you understand by the term 'non-stoichiometric hydrides'? Do you expect this type of hydrides to be formed by alkali metals? Justify your answer.

Accepted Answer

### What are Non-Stoichiometric Hydrides?

Non-stoichiometric hydrides, also known as interstitial hydrides or metallic hydrides, are compounds formed primarily by d-block and f-block elements (transition metals and inner transition metals) with hydrogen. The term 'non-stoichiometric' refers to the fact that the ratio of metal to hydrogen atoms in these compounds is not a simple whole number, and it can vary within a certain range, unlike typical stoichiometric compounds (e.g., H₂O, NaCl) where the elemental ratios are fixed.

These hydrides are formed when hydrogen atoms occupy the interstitial sites (voids) within the crystal lattice of the metal. The hydrogen atoms are small enough to fit into these spaces without significantly altering the metal's crystal structure. The extent of hydrogen absorption depends on factors like temperature and pressure.

**Key characteristics of non-stoichiometric hydrides:**
*   **Variable composition:** Their formulas are often represented as MHₓ, where 'x' is a non-integer and can vary (e.g., LaH₂.₈₇, TiH₁.₅₈-₁.₈₀, VH₀.₅₆).
*   **Metallic properties:** They often retain some metallic properties like electrical conductivity and luster, although these properties might be modified compared to the parent metal.
*   **Hard and brittle:** They are generally harder and more brittle than the parent metals.
*   **Hydrogen storage:** Many of these hydrides have the ability to absorb large volumes of hydrogen, making them potential candidates for hydrogen storage materials.
*   **Formation by d- and f-block metals:** They are typically formed by Group 3, 4, 5, 6, 7, 8, 9, 10, 11, and f-block elements. However, Group 7, 8, and 9 metals (e.g., Mn, Fe, Co, Ni) do not form hydrides under normal conditions, forming a 'hydride gap' in the periodic table.

### Do you expect this type of hydrides to be formed by alkali metals? Justify your answer.

No, we do not expect non-stoichiometric hydrides to be formed by alkali metals (Group 1 elements like Li, Na, K, Rb, Cs).

**Justification:**
Alkali metals form **ionic (saline) hydrides**, not non-stoichiometric (interstitial) hydrides. Here's why:

1.  **Electronegativity Difference:** Alkali metals are highly electropositive elements. When they react with hydrogen, there is a large electronegativity difference between the metal and hydrogen. This leads to the transfer of electrons from the metal to hydrogen, forming a metal cation (M⁺) and a hydride anion (H⁻). This results in the formation of a distinct ionic compound with a fixed, stoichiometric ratio (e.g., LiH, NaH, KH).

2.  **Crystal Structure and Bonding:** Ionic hydrides have a salt-like, crystalline structure (e.g., NaCl type for LiH, NaH, KH). The bonding is predominantly ionic, and the hydrogen exists as a discrete H⁻ ion. There are no 'interstitial sites' in the metallic lattice for hydrogen atoms to occupy in the same way as in transition metals.

3.  **Nature of Hydrogen:** In non-stoichiometric hydrides, hydrogen atoms are typically present in the interstitial voids of the metal lattice, often in a somewhat delocalized or covalent-like interaction with the metal atoms, without forming discrete H⁻ ions. In contrast, in alkali metal hydrides, hydrogen exists as a well-defined H⁻ ion.

4.  **Stoichiometry:** Alkali metal hydrides always exhibit a fixed stoichiometry of 1:1 (MH), meaning one metal atom combines with one hydrogen atom. This is a characteristic of stoichiometric compounds, not non-stoichiometric ones.

Therefore, the fundamental difference in bonding (ionic vs. interstitial/metallic) and the fixed stoichiometric composition prevent alkali metals from forming non-stoichiometric hydrides. They exclusively form stoichiometric ionic hydrides.

Question 20

Q20. How do you expect the metallic hydrides to be useful for hydrogen storage? Explain.

Accepted Answer

### Metallic Hydrides for Hydrogen Storage

Metallic hydrides (also known as interstitial or non-stoichiometric hydrides) are compounds formed by the reaction of hydrogen with certain transition metals and inner transition metals. These hydrides have garnered significant interest as potential materials for hydrogen storage due to their unique properties.

**Mechanism of Hydrogen Storage:**
1.  **Absorption:** When certain metals (e.g., Pd, Ti, V, La, Mg, Fe-Ti alloys) are exposed to hydrogen gas, hydrogen molecules (H₂) dissociate into individual hydrogen atoms (H) on the metal surface. These hydrogen atoms then diffuse into the metal lattice and occupy the interstitial sites (voids) within the crystal structure. This process is called absorption.
2.  **Hydride Formation:** The absorbed hydrogen atoms form weak bonds with the metal atoms, leading to the formation of a metallic hydride. The metal lattice expands slightly to accommodate the hydrogen atoms. The amount of hydrogen absorbed can be quite large, often exceeding the volume of the metal itself by hundreds or even thousands of times.
3.  **Desorption:** The absorbed hydrogen can be released from the metallic hydride by heating the material or by reducing the pressure. This process is called desorption, and it regenerates the metal and releases hydrogen gas.

**Why Metallic Hydrides are Useful for Hydrogen Storage:**

1.  **High Volumetric Hydrogen Density:** Metallic hydrides can store hydrogen at a much higher volumetric density than compressed hydrogen gas or even liquid hydrogen. For example, palladium can absorb up to 900 times its own volume of hydrogen, and some alloys can store even more. This is crucial for applications where space is limited, such as in vehicles.

2.  **Safety:** Storing hydrogen as a solid metallic hydride is inherently safer than storing it as a highly compressed gas or cryogenic liquid. The risk of leaks, explosions, or rapid dispersion in case of an accident is significantly reduced because hydrogen is chemically bound within the solid matrix and released only under specific conditions.

3.  **Reversibility:** Many metallic hydrides exhibit reversible absorption and desorption of hydrogen. This means they can be 'charged' with hydrogen and 'discharged' repeatedly without significant degradation, making them suitable for reusable storage systems.

4.  **Controllable Release:** The release of hydrogen from metallic hydrides can be controlled by varying temperature and pressure. This allows for on-demand hydrogen supply, which is advantageous for fuel cell applications.

5.  **Purity of Hydrogen:** The desorption process can sometimes yield very pure hydrogen, as impurities tend to remain bound to the metal or are not absorbed in the first place.

**Challenges and Future Directions:**
Despite these advantages, challenges remain, including:
*   **Weight:** Many metallic hydrides are heavy, which can be a disadvantage for mobile applications.
*   **Cost:** Some effective hydride-forming metals (e.g., palladium) are expensive.
*   **Kinetics:** The rates of hydrogen absorption and desorption can be slow for some materials.
*   **Thermodynamics:** The operating temperatures and pressures for efficient hydrogen release need to be optimized for practical applications.

Research is ongoing to develop new metallic hydride materials and alloys with improved storage capacity, faster kinetics, lower weight, and more favorable thermodynamics (i.e., releasing hydrogen at ambient temperatures and pressures) for widespread adoption in hydrogen economy applications, such as fuel cell vehicles and stationary power generation.

Question 21

Q21. What are the applications of heavy water?

Accepted Answer

Heavy water (D₂O) has several important applications due to its unique properties, primarily its higher mass compared to ordinary water (H₂O). Its main applications include:

1.  **Moderator in Nuclear Reactors:** This is the most significant application. Heavy water is an excellent moderator in nuclear reactors because deuterium has a very low neutron absorption cross-section. It slows down fast neutrons produced during nuclear fission, making them thermal neutrons, which are more effective in sustaining the chain reaction. Unlike ordinary water, it does not significantly absorb neutrons, thus allowing for more efficient use of natural uranium fuel.
2.  **Tracer Compound:** Due to the isotopic difference, heavy water can be used as a tracer to study reaction mechanisms, metabolic pathways in biological systems, and the exchange of hydrogen atoms in various chemical and biological processes.
3.  **Preparation of Deuterium Compounds:** Heavy water is a source of deuterium. Many deuterium-containing compounds (deuterated compounds) are prepared by reacting suitable substances with D₂O. For example, D₂SO₄, DCl, NaOD, etc., can be prepared. These deuterated compounds are often used in spectroscopy (e.g., NMR) and in studying reaction kinetics.
4.  **Solvent for NMR Spectroscopy:** In Nuclear Magnetic Resonance (NMR) spectroscopy, D₂O is often used as a solvent for proton NMR studies because deuterium nuclei (²H or D) do not produce a signal in the proton NMR region, thus avoiding interference with the signals from the sample protons.
5.  **Coolant in Nuclear Reactors:** Besides being a moderator, heavy water can also act as a coolant in certain types of nuclear reactors.

Question 22

Q22. Suggest a method to remove the temporary hardness of water.

Accepted Answer

Temporary hardness in water is caused by the presence of dissolved bicarbonates of calcium and magnesium (Ca(HCO₃)₂ and Mg(HCO₃)₂). These bicarbonates decompose upon heating to form insoluble carbonates, which precipitate out. There are two main methods to remove temporary hardness:

### 1. By Boiling

**Principle:** When water containing temporary hardness is boiled, the soluble bicarbonates decompose to form insoluble carbonates (for calcium) or hydroxides (for magnesium), which then precipitate out and can be removed by filtration or decantation.

**Process:**
*   When calcium bicarbonate is heated, it decomposes to form calcium carbonate, water, and carbon dioxide. Calcium carbonate is insoluble and precipitates out.
    Ca(HCO₃)₂(aq)  →(heat)  CaCO₃(s) ↓ + H₂O(l) + CO₂(g)
*   When magnesium bicarbonate is heated, it decomposes to form magnesium hydroxide, water, and carbon dioxide. Magnesium hydroxide is also insoluble and precipitates out.
    Mg(HCO₃)₂(aq)  →(heat)  Mg(OH)₂(s) ↓ + 2CO₂(g)

**Advantages:** This is a simple and effective method for small-scale water treatment, such as in homes.
**Disadvantages:** It is not suitable for large-scale industrial applications due to the energy cost involved in boiling large volumes of water.

### 2. Clark's Method

**Principle:** Clark's method involves adding a calculated amount of lime (calcium hydroxide, Ca(OH)₂) to the hard water. The lime reacts with the soluble bicarbonates to form insoluble calcium carbonate and magnesium hydroxide, which precipitate out.

**Process:**
*   When calcium hydroxide is added to water containing calcium bicarbonate, it reacts to form insoluble calcium carbonate, which precipitates.
    Ca(HCO₃)₂(aq) + Ca(OH)₂(aq) → 2CaCO₃(s) ↓ + 2H₂O(l)
*   When calcium hydroxide is added to water containing magnesium bicarbonate, it reacts to form insoluble magnesium hydroxide and calcium carbonate, both of which precipitate.
    Mg(HCO₃)₂(aq) + Ca(OH)₂(aq) → Mg(OH)₂(s) ↓ + CaCO₃(s) ↓ + H₂O(l)

**Advantages:** This method is more economical and suitable for large-scale water treatment compared to boiling, as it does not require heating.
**Disadvantages:** It requires careful control of the amount of lime added, as excess lime can itself cause hardness (though this is permanent hardness). The sludge (precipitate) needs to be disposed of.

Question 23

Q23. What do you understand by the term 'hydrogen economy'? Write the advantages of using hydrogen as a fuel.

Accepted Answer

### What is 'Hydrogen Economy'?

The term 'hydrogen economy' refers to a future vision of using hydrogen as a major energy carrier to power various sectors, including transportation, industry, and residential needs. In a hydrogen economy, hydrogen would replace fossil fuels (like coal, oil, and natural gas) as the primary source of energy. The concept involves producing hydrogen from various sources (preferably renewable ones like water electrolysis using solar or wind power), storing it, and then using it to generate electricity through fuel cells or by direct combustion.

The key idea is to create a sustainable energy infrastructure where hydrogen acts as a clean, efficient, and versatile energy carrier, reducing reliance on fossil fuels and mitigating environmental pollution, particularly greenhouse gas emissions.

### Advantages of Using Hydrogen as a Fuel

Using hydrogen as a fuel offers several significant advantages:

1.  **Clean Combustion Product:** The primary advantage is that hydrogen combustion or its use in fuel cells produces only water (H₂O) as a byproduct. This means zero greenhouse gas emissions (like CO₂) and no pollutants (like SOx, NOx, particulate matter) that are typically associated with fossil fuel combustion. This makes it an environmentally friendly fuel.
    2H₂(g) + O₂(g) → 2H₂O(l) + Energy
2.  **High Energy Content:** Hydrogen has the highest energy content per unit mass of any known fuel (approximately 143 kJ/g), which is about three times more than gasoline. This means a smaller mass of hydrogen can provide a large amount of energy.
3.  **Abundance:** Hydrogen is the most abundant element in the universe. While elemental hydrogen is rare on Earth, it is abundantly available in compounds like water (H₂O), from which it can be extracted. This ensures a potentially inexhaustible supply.
4.  **Versatility:** Hydrogen can be used in various ways:
    *   **Fuel Cells:** It can be converted directly into electricity in fuel cells with high efficiency, powering vehicles, homes, and portable devices.
    *   **Combustion:** It can be burned directly in internal combustion engines (with modifications) or gas turbines.
    *   **Industrial Feedstock:** It is already used in many industrial processes, such as ammonia production, petroleum refining, and methanol synthesis.
5.  **Renewable Production Potential:** Hydrogen can be produced from renewable sources like water using electrolysis powered by solar, wind, or hydroelectric energy. This makes it a sustainable energy carrier, unlike finite fossil fuels.
6.  **Reduced Noise Pollution:** Fuel cell vehicles powered by hydrogen are significantly quieter than traditional internal combustion engine vehicles, contributing to reduced noise pollution in urban areas.
7.  **Energy Storage:** Hydrogen can serve as a medium for storing energy. Excess electricity from intermittent renewable sources (like solar or wind) can be used to produce hydrogen, which can then be stored and used later when needed.

Question 24

Q24. Compare the structure of H₂O and H₂O₂ with the help of diagrams.

Accepted Answer

Both water (H₂O) and hydrogen peroxide (H₂O₂) are compounds of hydrogen and oxygen, but they exhibit distinct molecular structures due to the difference in the number of oxygen atoms and the resulting bonding arrangements.

### 1. Structure of Water (H₂O)

*   **Molecular Geometry:** Water has a **bent** or **V-shaped** molecular geometry.
*   **Hybridization:** The central oxygen atom is sp³ hybridized.
*   **Bonding:** Oxygen forms two single covalent bonds with two hydrogen atoms. It also has two lone pairs of electrons.
*   **Bond Angle:** The ideal bond angle for sp³ hybridization is 109.5°. However, due to the greater repulsion between the two lone pairs and the two bond pairs, the H-O-H bond angle is compressed to approximately **104.5°**.
*   **Planarity:** Water is a planar molecule.

### 2. Structure of Hydrogen Peroxide (H₂O₂)

*   **Molecular Geometry:** Hydrogen peroxide has a **non-planar, open book-like** structure. It is often described as having a 'half-open book' or 'gauche' conformation.
*   **Bonding:** There is an O-O single bond (peroxide linkage) and two O-H single bonds. Each oxygen atom is sp³ hybridized and has two lone pairs of electrons.
*   **Bond Angles:**
    *   The H-O-O bond angle is approximately **96.9°**.
    *   The O-O-H bond angle is approximately **96.9°**.
    *   The O-O bond length is about 147.5 pm.
    *   The O-H bond length is about 95.0 pm.
*   **Dihedral Angle:** The two O-H planes are not in the same plane. The dihedral angle (the angle between the two H-O-O planes) varies with the state:
    *   In the gas phase, it is approximately **111.5°**.
    *   In the solid phase, it is approximately **90.2°**.
*   **Planarity:** Hydrogen peroxide is a non-planar molecule.

### Comparison

**Diagrams:**

**(a) Structure of Water (H₂O)**
*   A central oxygen atom with two hydrogen atoms bonded to it, forming a bent shape. *   Two lone pairs of electrons should be shown on the oxygen atom, pushing the H-O-H bond angle to ~104.5°. *   The molecule should be depicted as planar.

**(b) Structure of Hydrogen Peroxide (H₂O₂)**
*   An O-O single bond in the center. *   One H atom bonded to each O atom. *   The two H-O-O planes should be shown at an angle to each other, resembling an open book. *   The dihedral angle between the two H-O-O planes should be indicated (e.g., 111.5° for gas phase). *   Bond angles (H-O-O ~96.9°) and bond lengths (O-O ~147.5 pm, O-H ~95.0 pm) should be labelled.

Question 25

Q25. What is the difference between the terms 'hydrolysis' and 'hydration'?

Accepted Answer

The terms 'hydrolysis' and 'hydration' both involve water, but they describe fundamentally different types of interactions or reactions:

### Hydrolysis

*   **Definition:** Hydrolysis (from Greek 'hydro' meaning water and 'lysis' meaning to unbind or break) is a **chemical reaction** in which water reacts with a substance, often leading to the **breaking of a chemical bond** in the substance and the formation of new compounds. Water itself is consumed in the reaction.
*   **Nature of Interaction:** It is a **chemical process** where water acts as a reactant, often involving the splitting of the water molecule (H-OH) into H⁺ and OH⁻ (or their equivalents) which then react with the substrate.
*   **Examples:**
    *   **Hydrolysis of salts:** When salts of weak acids and strong bases (e.g., sodium acetate) or strong acids and weak bases (e.g., ammonium chloride) dissolve in water, they undergo hydrolysis, affecting the pH of the solution.
        CH₃COONa(aq) + H₂O(l) ⇌ CH₃COOH(aq) + NaOH(aq)
        NH₄Cl(aq) + H₂O(l) ⇌ NH₄OH(aq) + HCl(aq)
    *   **Hydrolysis of esters:** Esters react with water (often in the presence of acid or base) to form a carboxylic acid and an alcohol.
        RCOOR' + H₂O ⇌ RCOOH + R'OH
    *   **Hydrolysis of ATP:** In biological systems, the hydrolysis of ATP releases energy.
        ATP + H₂O → ADP + Pi + Energy

### Hydration

*   **Definition:** Hydration is a **physical process** where water molecules surround and associate with ions or molecules, without necessarily breaking chemical bonds within the substance. It involves the formation of weak intermolecular forces (like ion-dipole or hydrogen bonding) between the water molecules and the solute.
*   **Nature of Interaction:** It is a **physical process** of solvation, where water acts as a solvent. It leads to the formation of hydrated species or hydrates, where water molecules are loosely bound to the solute.
*   **Examples:**
    *   **Hydration of ions:** When ionic compounds (like NaCl) dissolve in water, the individual ions (Na⁺ and Cl⁻) become surrounded by water molecules, forming hydrated ions (e.g., Na⁺(aq), Cl⁻(aq)). This process helps in the dissolution of ionic compounds.
    *   **Formation of hydrates:** Many salts crystallize from aqueous solutions with a definite number of water molecules attached to them, forming hydrates (e.g., CuSO₄·5H₂O, Na₂CO₃·10H₂O). The water molecules are part of the crystal structure.
    *   **Hydration of alkenes:** While it involves a chemical reaction, the term 'hydration' in organic chemistry often refers to the addition of water across a double bond (e.g., alkene to alcohol), where water itself is added as a molecule, not necessarily breaking its own bonds to react in the same way as hydrolysis.

### Key Differences Summarized

Question 26

Q26. What do you understand by the term 'non-stoichiometric hydrides'? Do you expect this type of hydrides to be formed by alkali metals? Justify your answer.

Accepted Answer

Non-stoichiometric hydrides are a class of hydrides that do not obey the law of constant proportions. This means that the ratio of hydrogen to the metal atom in these compounds is not a simple whole number, and their composition can vary within certain limits. They are also known as interstitial hydrides because hydrogen atoms occupy interstitial sites (voids) in the crystal lattice of the metal. These hydrides are typically formed by d-block and f-block elements.

**Characteristics of Non-stoichiometric Hydrides:**
1.  **Variable Composition:** Their formula is often represented as MHx, where x is not an integer and can vary, e.g., TiH1.5-1.8, ZrH1.3-1.7, VH0.56.
2.  **Metallic Properties:** They often retain the metallic luster, electrical conductivity, and thermal conductivity of the parent metals.
3.  **Hardness and Brittleness:** They are generally harder and more brittle than the parent metals.
4.  **Density:** Their density is usually lower than that of the parent metal.

**Formation by Alkali Metals:**
No, we do not expect non-stoichiometric hydrides to be formed by alkali metals (Group 1 elements).

**Justification:**
Alkali metals (Li, Na, K, Rb, Cs) are highly electropositive elements. When they react with hydrogen, they form **ionic hydrides** (also known as saline hydrides). In these hydrides, the alkali metal donates an electron to hydrogen, forming a metal cation (M+) and a hydride anion (H-). The resulting compounds are stoichiometric, meaning they have a fixed and definite composition, typically MH (e.g., LiH, NaH, KH).

**Reasons why alkali metals do not form non-stoichiometric hydrides:**
1.  **Ionic Bonding:** Alkali metals form predominantly ionic bonds with hydrogen, leading to discrete M+ and H- ions arranged in a crystal lattice. This type of bonding does not favor the interstitial accommodation of hydrogen atoms in a variable ratio.
2.  **Lack of d-orbitals:** Alkali metals do not possess vacant d-orbitals that are characteristic of transition metals. The ability to form non-stoichiometric hydrides is often associated with the presence of vacant d-orbitals in transition metals, which allows for the formation of metallic bonding and the accommodation of hydrogen in interstitial sites without significantly altering the metal's crystal structure.
3.  **Fixed Valency:** Alkali metals exhibit a fixed valency of +1, leading to a definite stoichiometric ratio with hydrogen (1:1).

Therefore, alkali metals form stoichiometric ionic hydrides, not non-stoichiometric metallic/interstitial hydrides.

Question 27

Q27. How do you expect the metallic hydrides to be useful for hydrogen storage? Explain.

Accepted Answer

Metallic hydrides, particularly those formed by transition metals, d-block, and f-block elements, show great promise for hydrogen storage due to their unique ability to absorb and desorb large volumes of hydrogen. This property makes them potential candidates for safe and efficient hydrogen storage in various applications, especially in the context of a hydrogen economy.

**Mechanism of Hydrogen Storage in Metallic Hydrides:**
1.  **Absorption (Hydrogenation):** When certain metals or alloys (e.g., Pd, LaNi5, FeTi, Mg2Ni) are exposed to hydrogen gas, hydrogen molecules (H2) dissociate on the metal surface into hydrogen atoms (H). These hydrogen atoms then diffuse into the metal's crystal lattice, occupying interstitial sites (voids) and forming a metallic hydride. This process is often exothermic.
    *   M(s) + x/2 H2(g) ⇌ MHx(s) + Heat
    The hydrogen atoms are chemically bonded to the metal atoms, but the overall structure often retains some metallic character.

2.  **Desorption (Dehydrogenation):** The absorbed hydrogen can be released from the metallic hydride by applying heat or reducing the pressure. This process is endothermic, and the hydrogen atoms recombine to form H2 molecules, which then escape from the metal lattice.
    *   MHx(s) + Heat ⇌ M(s) + x/2 H2(g)

**Advantages of Metallic Hydrides for Hydrogen Storage:**
1.  **High Volumetric Density:** Metallic hydrides can store hydrogen at a much higher volumetric density than compressed hydrogen gas or even liquid hydrogen. For example, some hydrides can store more hydrogen per unit volume than liquid hydrogen, making them highly efficient in terms of space utilization.
2.  **Safety:** Hydrogen stored in metallic hydrides is much safer than compressed or liquid hydrogen. It is stored in a solid, non-volatile form, significantly reducing the risks of explosion or leakage associated with high-pressure gas cylinders or cryogenic liquid tanks.
3.  **Reversibility:** The absorption and desorption processes are reversible, allowing the hydrides to be charged and discharged with hydrogen multiple times without significant degradation.
4.  **Moderate Operating Conditions:** Hydrogen can be absorbed and released under relatively moderate temperature and pressure conditions, making the technology practical for various applications.
5.  **Purity:** The desorption process can sometimes act as a purification step, as only hydrogen is released, leaving impurities behind.

**Challenges and Future Directions:**
Despite their advantages, challenges remain, such as the weight of the hydride materials (gravimetric density), the kinetics of absorption/desorption, and the cost. Research is ongoing to develop new hydride materials with improved gravimetric density, faster kinetics, and lower operating temperatures for widespread adoption in fuel cell vehicles, stationary power generation, and portable electronic devices.

Question 28

Q28. What is the reaction of H2O2 with KMnO4 in acidic medium?

Accepted Answer

In acidic medium, hydrogen peroxide (H2O2) acts as a reducing agent, while potassium permanganate (KMnO4) acts as a strong oxidizing agent. The purple MnO4- ion is reduced to the colorless Mn2+ ion, and H2O2 is oxidized to oxygen gas (O2).

**Ionic Equation:**
2MnO4-(aq) + 5H2O2(aq) + 6H+(aq) → 2Mn2+(aq) + 8H2O(l) + 5O2(g)

**Molecular Equation (if KMnO4 is used with H2SO4 as acid):**
2KMnO4(aq) + 5H2O2(aq) + 3H2SO4(aq) → K2SO4(aq) + 2MnSO4(aq) + 8H2O(l) + 5O2(g)

**Explanation:**
*   **Oxidation Half-reaction (H2O2):** H2O2 → O2 + 2H+ + 2e-
    (Oxidation state of oxygen changes from -1 in H2O2 to 0 in O2)
*   **Reduction Half-reaction (MnO4-):** MnO4- + 8H+ + 5e- → Mn2+ + 4H2O
    (Oxidation state of manganese changes from +7 in MnO4- to +2 in Mn2+)

To balance the overall reaction, the oxidation half-reaction is multiplied by 5 and the reduction half-reaction by 2, then added together.

Question 29

Q29. What is meant by 'demineralised' water and how can it be obtained?

Accepted Answer

Demineralised water, also known as deionized water, is water that has had almost all of its mineral ions, such as cations (Na+, Ca2+, Mg2+, Fe3+) and anions (Cl-, SO42-, HCO3-, NO3-), removed. It is essentially pure water, free from dissolved salts, but it may still contain non-ionic impurities like dissolved gases or organic matter. Demineralised water is distinct from distilled water, which is produced by boiling and condensation, and removes both ionic and non-ionic impurities (though less efficiently for some dissolved gases).

**How Demineralised Water Can Be Obtained (Ion-Exchange Process):**
Demineralised water is typically obtained using an ion-exchange process, which involves passing water through a series of ion-exchange resins. This process effectively removes all dissolved ionic impurities.

**The Ion-Exchange Process involves two main types of resins:**

1.  **Cation Exchange Resin (H-form):**
    *   This resin contains acidic functional groups (e.g., -SO3H) with exchangeable H+ ions. When hard water (containing metal cations like Ca2+, Mg2+, Na+) passes through this resin, the metal cations are exchanged for H+ ions.
    *   **Reaction:**
        2R-H(s) + M2+(aq) → R2M(s) + 2H+(aq)
        (where R represents the resin matrix, M2+ is a metal cation like Ca2+ or Mg2+)
        R-H(s) + Na+(aq) → R-Na(s) + H+(aq)
    *   The water leaving this column now contains H+ ions and the original anions.

2.  **Anion Exchange Resin (OH-form):**
    *   This resin contains basic functional groups (e.g., -NR3+OH-) with exchangeable OH- ions. The water, now containing H+ ions and various anions (e.g., Cl-, SO42-), then passes through this column. The anions are exchanged for OH- ions.
    *   **Reaction:**
        R-OH(s) + X-(aq) → R-X(s) + OH-(aq)
        (where X- is an anion like Cl-, SO42-, HCO3-)
    *   The H+ ions released from the cation exchanger combine with the OH- ions released from the anion exchanger to form water (H+ + OH- → H2O).

**Overall Process:**
*   Raw water first passes through the cation exchange column, where all cations are replaced by H+ ions.
*   The effluent from the cation exchanger then passes through the anion exchange column, where all anions are replaced by OH- ions.
*   The H+ and OH- ions combine to form pure water.

**Regeneration of Resins:**
Over time, the resins become saturated with the exchanged ions and lose their effectiveness. They need to be regenerated:
*   **Cation exchange resin:** Regenerated by passing a dilute acid (e.g., HCl or H2SO4) through it, which replaces the adsorbed metal ions with H+ ions.
    *   R2M(s) + 2H+(aq) → 2R-H(s) + M2+(aq)
*   **Anion exchange resin:** Regenerated by passing a dilute base (e.g., NaOH) through it, which replaces the adsorbed anions with OH- ions.
    *   R-X(s) + OH-(aq) → R-OH(s) + X-(aq)

This two-step ion-exchange process produces highly pure demineralised water, suitable for laboratory use, industrial processes, and in applications where mineral-free water is critical (e.g., in batteries, steam irons, pharmaceutical manufacturing).

Question 30

Q30. Can we use concentrated sulphuric acid and anhydrous calcium chloride for drying hydrogen gas? Justify your answer.

Accepted Answer

No, we cannot use concentrated sulphuric acid (conc. H2SO4) and anhydrous calcium chloride (anhydrous CaCl2) for drying hydrogen gas.

**Justification:**

1.  **Concentrated Sulphuric Acid (conc. H2SO4):**
    *   Concentrated sulphuric acid is a strong oxidizing agent. Although hydrogen gas is generally unreactive, it can react with hot concentrated sulphuric acid, especially if there are impurities or if the acid is very concentrated and hot.
    *   The reaction would lead to the oxidation of hydrogen and the reduction of sulphuric acid, thus not only failing to dry the hydrogen but also consuming it and introducing impurities.
    *   H2(g) + H2SO4(conc.) → SO2(g) + 2H2O(l) (This reaction typically requires heating or specific conditions, but the risk exists).

2.  **Anhydrous Calcium Chloride (anhydrous CaCl2):**
    *   Anhydrous calcium chloride is an excellent drying agent, but it forms an addition compound (CaCl2·nH2) with hydrogen gas, particularly at lower temperatures.
    *   This means that CaCl2 would react with hydrogen, absorbing it rather than just drying it, and thus cannot be used for drying hydrogen gas.

**Suitable Drying Agents for Hydrogen Gas:**
For drying hydrogen gas, common drying agents that do not react with hydrogen are used. These include:
*   Anhydrous phosphorus pentoxide (P4O10)
*   Fused calcium chloride (CaCl2) (Note: anhydrous CaCl2 is problematic, but fused CaCl2 is sometimes used with caution, though P4O10 is preferred for very dry H2)
*   Silica gel
*   Alumina (Al2O3)

Therefore, due to their reactivity with hydrogen, concentrated sulphuric acid and anhydrous calcium chloride are unsuitable for drying hydrogen gas.

Question 31

Q31. What do you understand by the term 'non-stoichiometric hydrides'? Do these hydrides have the same composition as expected from their parent metals? Justify your answer.

Accepted Answer

### Non-stoichiometric Hydrides

Non-stoichiometric hydrides, also known as interstitial hydrides or metallic hydrides, are formed by many d-block and f-block elements. These hydrides are characterized by their variable composition, which means the ratio of metal to hydrogen atoms is not a simple whole number and often deviates from the expected stoichiometric formula based on the metal's valency.

### Composition and Deviation from Expected Stoichiometry

No, these hydrides do not have the same composition as expected from their parent metals based on simple stoichiometric rules.

**Justification:**

1.  **Interstitial Nature:** Transition metals and inner transition metals have vacant d-orbitals and f-orbitals, respectively, and relatively large atomic sizes. This allows hydrogen atoms to occupy the interstitial sites (voids) within the metal lattice without forming distinct covalent or ionic bonds in a fixed ratio. The extent of hydrogen absorption depends on factors like temperature and pressure.
2.  **Variable Composition:** The amount of hydrogen absorbed by the metal can vary significantly, leading to non-stoichiometric formulas. For example, titanium forms TiH1.5-1.8, zirconium forms ZrH1.3-1.75, and vanadium forms VH0.56. These formulas clearly show that the hydrogen content is not a fixed integer, unlike typical chemical compounds.
3.  **Defect Structures:** The formation of these hydrides often involves the creation of defects in the metal lattice, and the hydrogen atoms occupy these interstitial positions. The exact composition can depend on the conditions of preparation.
4.  **Absence of Group 7, 8, 9 Elements:** Interestingly, metals of Group 7, 8, and 9 (e.g., Mn, Fe, Co, Ni) do not form hydrides. This region of the periodic table is known as the 'hydride gap'. This further supports the idea that hydride formation in transition metals is not always straightforwardly stoichiometric.

In summary, non-stoichiometric hydrides are characterized by their variable composition due to hydrogen occupying interstitial sites in the metal lattice, leading to formulas that deviate from simple whole-number ratios expected from the metal's valency.

Question 32

Q32. How do you expect the metallic hydrides to be useful for hydrogen storage? Explain.

Accepted Answer

Metallic hydrides are considered promising materials for hydrogen storage due to their unique properties that allow them to absorb and release hydrogen reversibly. This makes them a safer and more compact alternative to storing hydrogen as a compressed gas or cryogenic liquid.

### Usefulness of Metallic Hydrides for Hydrogen Storage:

1.  **High Hydrogen Density:** Metallic hydrides can store a significantly higher density of hydrogen compared to gaseous or even liquid hydrogen at ambient conditions. Hydrogen atoms occupy interstitial sites within the metal lattice, leading to a compact storage solution. For example, some hydrides can store more hydrogen per unit volume than liquid hydrogen.

2.  **Reversible Absorption and Desorption:** Many metallic hydrides exhibit reversible hydrogen absorption and desorption. Under certain temperature and pressure conditions, the metal can absorb hydrogen to form the hydride (e.g., M + x/2 H2 → MHx). By changing these conditions (e.g., increasing temperature or decreasing pressure), the hydride can release hydrogen (e.g., MHx → M + x/2 H2). This reversibility is crucial for practical storage applications where hydrogen needs to be stored and then released on demand.

3.  **Safety:** Storing hydrogen as a solid hydride is inherently safer than storing it as a highly flammable compressed gas or cryogenic liquid. The hydrogen is chemically bound within the metal lattice, reducing the risk of leaks, explosions, or rapid dispersion in case of an accident.

4.  **Moderate Operating Conditions:** While some hydrides require specific temperature and pressure conditions, many can operate at relatively moderate temperatures and pressures, making them suitable for various applications, including automotive and stationary power generation.

5.  **Purity of Hydrogen:** The desorption process from metallic hydrides can often yield high-purity hydrogen, as the metal lattice acts as a filter, leaving impurities behind.

**Explanation of Mechanism:**

When hydrogen gas comes into contact with certain transition metals (e.g., palladium, lanthanum, magnesium, titanium alloys), it dissociates into atomic hydrogen on the metal surface. These hydrogen atoms then diffuse into the bulk of the metal and occupy the interstitial sites within the crystal lattice, forming a metallic hydride. This process is exothermic. The amount of hydrogen absorbed depends on the metal, temperature, and pressure. When heat is supplied or the pressure is reduced, the hydrogen atoms recombine and are released as H2 gas, regenerating the metal.

**Examples of materials being researched:**
*   MgH2 (Magnesium hydride)
*   LaNi5H6 (Lanthanum nickel hydride)
*   TiFeH2 (Titanium iron hydride)
*   Complex hydrides like NaAlH4 (Sodium aluminium hydride) and LiBH4 (Lithium borohydride) are also being investigated for their high hydrogen storage capacity.

In conclusion, the ability of metallic hydrides to store hydrogen at high densities, reversibly, and safely under relatively mild conditions makes them a highly promising technology for future hydrogen-based energy systems.

Question 33

Q33. What is the difference between the terms 'hydrolysis' and 'hydration'?

Accepted Answer

The terms 'hydrolysis' and 'hydration' both involve water but describe fundamentally different chemical processes.

**Hydrolysis:**
*   **Definition:** Hydrolysis is a chemical reaction in which water (H2O) reacts with a substance, causing the breaking of one or more chemical bonds in that substance. The water molecule itself is consumed and often splits into H+ and OH- fragments, which then become incorporated into the products.
*   **Nature of Reaction:** It is a **chemical reaction** where water acts as a reactant, leading to the formation of new chemical compounds.
*   **Examples:**
    *   Hydrolysis of an ester: RCOOR' + H2O → RCOOH + R'OH (ester breaks into carboxylic acid and alcohol)
    *   Hydrolysis of salts: Na2CO3 + 2H2O ⇌ H2CO3 + 2NaOH (carbonate ion reacts with water to form carbonic acid and hydroxide ions, making the solution basic)
    *   Hydrolysis of ATP in biological systems.

**Hydration:**
*   **Definition:** Hydration is the process where water molecules surround and interact with ions or polar molecules, forming a complex or an aggregate. It typically involves the formation of weak intermolecular forces (like hydrogen bonds or ion-dipole interactions) between water and the solute, without breaking the internal bonds of the solute.
*   **Nature of Reaction:** It is primarily a **physical process** or a solvation process, where water acts as a solvent. While it can involve energy changes (hydration enthalpy), it generally does not lead to the formation of new chemical compounds by breaking the solute's bonds.
*   **Examples:**
    *   Dissolving an ionic compound in water: Na+(aq) and Cl-(aq) ions are surrounded by water molecules.
    *   Hydration of anhydrous copper(II) sulfate: CuSO4(s) + 5H2O(l) → CuSO4·5H2O(s) (water molecules are incorporated into the crystal lattice as water of crystallization, but the CuSO4 molecule itself doesn't break apart).
    *   Hydration of alkenes to form alcohols (this is a specific type of addition reaction where water adds across a double bond, but the general concept of hydration refers to solvation).

**Key Differences Summarized:**

Question 34

Q34. How can saline hydrides remove traces of water from organic compounds? Explain with chemical reaction.

Accepted Answer

Saline hydrides, also known as ionic hydrides, are formed by the reaction of hydrogen with highly electropositive s-block elements (Group 1 and Group 2 metals). Examples include LiH, NaH, CaH2, BaH2. These hydrides are salt-like, crystalline, non-volatile solids with high melting points. Their key characteristic for drying purposes is their strong reactivity with water.

### Mechanism of Water Removal by Saline Hydrides

Saline hydrides are powerful drying agents because they react vigorously and irreversibly with water to produce hydrogen gas and the corresponding metal hydroxide. This reaction effectively removes water from the system.

Consider Calcium Hydride (CaH2) as a common example used for drying organic compounds:

1.  **Reaction with Water:** When calcium hydride comes into contact with traces of water present in an organic solvent or compound, it reacts immediately and completely with the water molecules.

**Chemical Reaction:**
    CaH2(s) + 2H2O(l) → Ca(OH)2(s) + 2H2(g)

*   In this reaction, the hydride ion (H-) from CaH2 acts as a strong base and reduces water. It accepts a proton from water to form H2 gas, and the remaining OH- combines with Ca2+ to form calcium hydroxide.
    *   The formation of gaseous hydrogen (H2) and solid calcium hydroxide (Ca(OH)2) ensures that water is permanently removed from the organic phase. Both products are easily separable from the organic compound.

2.  **Irreversible and Complete Removal:** The reaction is highly exothermic and irreversible, driving it to completion and ensuring efficient removal of even trace amounts of water.

3.  **Non-reactive with Organic Compounds:** Saline hydrides generally do not react with most common organic compounds (especially those without acidic protons or highly reactive functional groups like aldehydes/ketones, esters, etc., which might be reduced by H-). This selectivity makes them suitable for drying sensitive organic solvents and compounds.

### Advantages as a Drying Agent:
*   **High Efficiency:** Due to the vigorous and complete reaction with water.
*   **Versatility:** Can be used for drying a wide range of organic solvents (e.g., ethers, hydrocarbons, halogenated solvents).
*   **Visual Indicator:** The evolution of hydrogen gas can sometimes serve as an indicator of the presence of water.

Therefore, saline hydrides like CaH2 are effective desiccants for removing traces of water from organic compounds by chemically converting water into hydrogen gas and a metal hydroxide, which are easily separated from the desired organic substance.

Question 35

Q35. What is the difference between the terms 'hydrolysis' and 'hydration'?

Accepted Answer

The terms 'hydrolysis' and 'hydration' both involve water but describe fundamentally different processes:

**Hydrolysis:**
1.  **Definition:** Hydrolysis is a chemical reaction in which water (H₂O) reacts with a compound, leading to the breaking of one or more chemical bonds in the compound. The water molecule itself is often split into H⁺ and OH⁻, which then become incorporated into the products.
2.  **Nature:** It is a chemical change, resulting in the formation of new chemical substances.
3.  **Mechanism:** Involves the breaking of existing bonds and the formation of new covalent bonds.
4.  **Examples:**
    *   Hydrolysis of an ester: RCOOR' + H₂O → RCOOH + R'OH (acid + alcohol)
    *   Hydrolysis of a salt of a weak acid and strong base (e.g., sodium acetate): CH₃COONa + H₂O ⇌ CH₃COOH + NaOH
    *   Hydrolysis of ATP in biological systems.
    *   Hydrolysis of non-metal halides (e.g., PCl₃ + 3H₂O → H₃PO₃ + 3HCl).

**Hydration:**
1.  **Definition:** Hydration is a physical process in which water molecules surround and interact with ions or polar molecules of a solute without breaking the chemical bonds within the solute. It is a type of solvation where water acts as the solvent.
2.  **Nature:** It is primarily a physical process, leading to the formation of a hydrated species or a solution. Chemical bonds within the solute are generally not broken.
3.  **Mechanism:** Involves intermolecular forces such as ion-dipole interactions (for ionic compounds), hydrogen bonding, or dipole-dipole interactions (for polar covalent compounds) between water molecules and the solute.
4.  **Examples:**
    *   Hydration of an anhydrous salt (e.g., CuSO₄ + 5H₂O → CuSO₄·5H₂O, where water molecules are coordinated to the copper ion and sulfate).
    *   Hydration of ions in an aqueous solution (e.g., Na⁺(aq), Cl⁻(aq)).
    *   Hydration of alkenes to form alcohols (though this involves a chemical reaction, the initial interaction of water with the double bond can be considered a form of hydration).
    *   Dissolving sugar in water (sugar molecules are hydrated by water molecules).

**Summary of Differences:**

Question 36

Q36. How can saline, glassy and metallic hydrides be distinguished from each other?

Accepted Answer

The question refers to 'glassy' hydrides, which is not a standard classification. It is likely a typo and refers to 'covalent' or 'molecular' hydrides, which can sometimes be amorphous or glassy in solid form. Therefore, we will distinguish between Saline (Ionic), Covalent (Molecular), and Metallic (Interstitial) hydrides.

### Distinction between Saline, Covalent, and Metallic Hydrides

#### 1. Saline Hydrides (Ionic Hydrides)

*   **Formation:** Formed by s-block elements (Group 1 and Group 2, except Be and Mg which form polymeric hydrides). Examples: NaH, CaH₂.
*   **Nature:** These are stoichiometric compounds, typically crystalline, non-volatile solids at room temperature.
*   **Bonding:** Primarily ionic, with hydrogen existing as a hydride ion (H⁻).
*   **Physical Properties:**
    *   White, crystalline solids.
    *   High melting and boiling points.
    *   Do not conduct electricity in the solid state but are good conductors in the molten state or in aqueous solution (due to the presence of mobile ions, H⁻ and metal cations).
    *   Denser than the parent metal.
*   **Chemical Properties:**
    *   React violently with water to produce dihydrogen gas and the corresponding metal hydroxide: M⁺H⁻(s) + H₂O(l) → M⁺OH⁻(aq) + H₂(g).
    *   Strong reducing agents.
    *   Electrolysis of molten saline hydrides yields H₂ gas at the anode (2H⁻ → H₂ + 2e⁻) and metal at the cathode.
*   **Examples:** LiH, NaH, KH, RbH, CsH, CaH₂, SrH₂, BaH₂.

#### 2. Covalent Hydrides (Molecular Hydrides)

*   **Formation:** Formed by p-block elements (and some s-block like BeH₂ and MgH₂ which are polymeric, and B₂H₆ from Group 13). Examples: CH₄, NH₃, H₂O, HCl.
*   **Nature:** These are typically volatile compounds (gases or liquids) at room temperature, though some can be solids (e.g., H₂O, HF). They are generally non-crystalline or can form amorphous/glassy solids upon rapid cooling.
*   **Bonding:** Covalent, with hydrogen sharing electrons with the non-metal atom.
*   **Physical Properties:**
    *   Low melting and boiling points (due to weak intermolecular forces).
    *   Poor conductors of electricity in all states.
    *   Can be gases, liquids, or low-melting solids.
*   **Chemical Properties:**
    *   Their reactivity varies widely depending on the central atom's electronegativity and size.
    *   Can be electron-deficient (e.g., B₂H₆), electron-precise (e.g., CH₄), or electron-rich (e.g., NH₃, H₂O, HF).
    *   Electron-deficient hydrides act as Lewis acids.
    *   Electron-rich hydrides (with lone pairs) act as Lewis bases.
    *   Some are acidic (e.g., HCl, H₂S), some are basic (e.g., NH₃), and some are neutral (e.g., CH₄).
*   **Examples:** CH₄, NH₃, H₂O, HF, H₂S, SiH₄, PH₃.

#### 3. Metallic Hydrides (Interstitial Hydrides)

*   **Formation:** Formed by d-block and f-block elements. Group 7, 8, 9 elements generally do not form hydrides (this region is known as the 'hydride gap'). Examples: TiH₁.₇₃, LaH₂.₈₇.
*   **Nature:** These are typically non-stoichiometric compounds (except for Group 3 and some Group 4 elements), meaning the ratio of metal to hydrogen is not a simple whole number. They retain metallic lustre and properties.
*   **Bonding:** Hydrogen atoms occupy interstitial sites (voids) within the metal lattice. The bonding is primarily metallic, with some covalent character.
*   **Physical Properties:**
    *   Possess metallic properties like lustre, hardness, and electrical conductivity (though often less conductive than the parent metal).
    *   Often less dense than the parent metal.
    *   Can absorb large volumes of hydrogen, making them useful for hydrogen storage.
*   **Chemical Properties:**
    *   Generally less reactive than saline hydrides.
    *   Can absorb and desorb hydrogen reversibly, which is crucial for hydrogen storage and purification.
    *   Their exact properties depend on the specific metal and hydrogen content.
*   **Examples:** TiHₓ, ZrHₓ, VHₓ, LaHₓ, PdHₓ.

### Summary Table for Distinction

Chapter 9: Hydrogen